Thought I'd share one of my recent experiments - a capacitive proximity sensor built into the arm. I've been wanting to try this out for a while and it works pretty well as a proof-of-concept. There have been small projects on proximity sensing here and there but I haven't seen it actually used in an arm. It makes for a simple and cheap alternative to actual torque or force sensing.
The concept itself is pretty simple and can be easily implemented with https://playground.arduino.cc/Main/CapacitiveSensor/. For the capacitive plate, I used alternating layers of copper tape and aluminium foil, about 3 to 4 layers of each, along with a 40MOhm resistor.
Unfortunately, I wasn't to get much working beyond this due to the EMI from the motor cables (this is with only J1 powered). I'm not really up for re-wiring the arm so I'm putting this idea aside for now. Still, I think it's a really cool concept. If anyone else gets to properly shielding their wiring and takes this further, I'll be interested to see the results!